The present invention relates to a sub-sea choke valve having pressure sensing transmitters.
A choke valve is a throttling device. It is commonly used as part of an oil or gas field wellhead. It functions to reduce the pressure of the fluid flowing through the valve. Choke valves are placed on the production xe2x80x9ctreexe2x80x9d of an oil or gas wellhead assembly to control the flow of produced fluid from a reservoir into the production flow line. They are used on wellheads located on land and on off-shore wellheads located beneath the surface of the ocean.
A choke valve incorporates what is referred to as a xe2x80x9cflow trimxe2x80x9d. The flow trim is positioned within the choke valve at the intersection of the choke valve""s inlet and outlet. The flow trim commonly comprises a stationary tubular cylinder referred to as a xe2x80x9ccagexe2x80x9d. The cage is positioned transverse to the inlet and its bore is axially aligned with the outlet. The cage has restrictive flow ports extending through its sidewall. Fluid enters the cage from the choke valve inlet, passes through the ports and changes direction to leave the cage bore through the valve outlet.
Such a flow trim also comprises a tubular throttling sleeve that slides over the cage. The sleeve acts to reduce or increase the area of the ports. An actuator, such as a threaded stem assembly, is provided to bias the sleeve back and forth along the cage. The amount of fluid that passes through the flow trim is dependent on the relative position of the sleeve on the cage and the amount of port area that is revealed by the sleeve.
Sub-sea oil or gas wells can be as deep as 6000 feet below sea level. At these depths, maintenance on the wellhead assemblies cannot be performed manually. An unmanned, remotely operated vehicle, referred to as an xe2x80x9cROVxe2x80x9d, is used to approach the wellhead and carry out maintenance functions. To aid in servicing sub-sea choke valves, choke valves have their internal components, including the flow trim, assembled into a modular sub-assembly. The sub-assembly is referred to as an xe2x80x9cinsert assemblyxe2x80x9d and is inserted into the choke valve body and clamped into position.
A typical prior art sub-sea choke valve a is shown in FIG. 1. It comprises a choke body b forming a T-shaped bore c that provides a horizontal inlet d, a vertical bottom outlet e and an upper vertical component chamber f. A removable insert assembly g is positioned in the component chamber f, extending transversely of the inlet d. The insert assembly g comprises a tubular cartridge h, forming a side port i, a flow trim j comprising a cage k and throttling sleeve 1, a collar assembly m and a bonnet n. The bonnet n is disengagably clamped to the body b. It closes the upper ends of the valve body b and the cartridge h. The stem assembly m extends through the bonnet n into the cartridge bore o to bias the sleeve 1 along the cage k to throttle the restrictive flow ports p.
The choke valve xe2x80x98seesxe2x80x99 or experiences relatively high and relatively low fluid pressures. More particularly, the fluid flowing in through the valve inlet d from the well (not shown) has a high pressure. When the fluid passes through the restrictive cage ports p, it undergoes a considerable pressure drop. So the fluid passing through the cage bore q and the valve outlet e is at lower pressure than that in the body inlet d.
The high pressure fluid penetrates into an annular clearance r between the cartridge h and the internal bore surface s of the body b. Also, the low pressure fluid in the cage bore q and valve outlet e penetrates through communication ports t in the end wall of the throttling sleeve 1 and into a passageway u extending partway along the length of the collar assembly m.
When the flow trim j becomes worn beyond its useful service life due to erosion and corrosion caused by particles and corrosive agents in the produced substances, an ROV is used to approach the choke valve a, unclamp the insert assembly g from the choke valve body b and attach a cable to the insert assembly g, so that it may be raised to the surface for replacement or repair. The ROV then installs a new insert assembly and clamps it into position. This procedure eliminates the need to raise the whole wellhead assembly to the surface to service a worn choke valve.
In order to efficiently produce a reservoir, it is necessary to monitor the pressure upstream and downstream of the choke valve. This is done to ensure that damage to the formation does not occur and to ensure that well production is maximized. This process has been, historically, accomplished through the installation of pressure transmitters into the flow lines upstream and downstream of the choke valve. The pressure read by the upstream and downstream pressure transmitters is sent to a remote location for monitoring, so that a choke valve controller can remotely bias the flow trim to affect the desired downstream flow line pressure. The controller sends electrical signals to means, associated with the choke valve, for adjusting the flow trim.
A problem, however, exists with this process due to the unreliable nature of these electronic pressure transmitters, which have a limited service life. Replacing the pressure transmitters after they have served their useful life has heretofore required that the whole wellhead assembly be raised to the surface. This is a time-consuming and costly operation that shuts down well production for the duration of the repair.
The present invention proposes a modified choke valve that eliminates the need to raise a sub-sea wellhead assembly to the surface to replace or repair pressure transmitters.
The invention involves locating one or preferably a pair of pressure transmitters in the upper end of the bonnet of the insert assembly. One transmitter is connected by a first passageway extending through the bonnet to communicate with the annular clearance between the cartridge and the valve body. This transmitter thus can measure the high pressure of the incoming fluid and transmit a signal indicative thereof to a remote receiver. A second transmitter is connected by a second passageway, extending through the bonnet, which communicates with the valve outlet. This transmitter thus can measure the pressure of the fluid that has undergone a pressure drop by passing through the flow trim. It can transmit a signal indicative of this reduced pressure to the remote receiver.
By positioning one or both of the transmitters in the bonnet of the removable insert assembly, they can now be replaced or repaired economically by bringing the choke insert assembly to surface.